It may have bun spun into a Hollywood tale but the idea of a designated survivor is in fact very real. Each year during the state of the union address, there is one person who is ready to assume the presidency in the event our government is wiped out. What's it like to be a real-life designated survivor? Here's my "Nightline" coanchor juju Chang. Reporter: It is the unthinkable. The doomsday scenario. A catastrophic attack on U.S. Soil. The president, congress, the upper levels of government, all wiped up. While it's the stuff of dark thrillers, the creators of ABC's most-talked about show "Designated survivor" have something very real in mind. Sir, you are now the president of the United States. So this is your presidential motorcade? This is a small one. Normally there's 16 cars. Kiefer Sutherland playing a meek, low-level cabinet secretary. Who's thrust full Yao unprepared into the presidency. So help me god. Reporter: Though firmly in the realm of make-believe it's based on a very real, extremely classified safeguard known as the continuity of government plan. The problem that the continuity of government plan tries to solve is something called decapitation. Reporter: ABC news consultant and former white house counterterrorism official dick Clark ran the program for nearly a decade. He's the only official to ever deploy it in a crisis, during 9/11. While president bush was in the air, vice president Cheney was rushed to an underground bunker. On 9/11 I activated the continuity of government system. We asked the speaker of the house to leave Washington. Reporter: The government didn't know if more attacks were coming and where. Gail Norton, George W. Bush's secretary of the interior, told "20/20" anchor Elizabeth vargas she was one of a handful of high-ranking officials sent to secret locations. I know you can't tell me exactly where you were but can you describe the conditions? The shampoo had curdled. The deodorant smelled bad. Because it was all so old. Reporter: She was pressed into duty as the first state of the union designated survivor. As we gather tonight, our nation is at war, the economy is in recession, and the civilized world faces unprecedented dangers. Because we had seen what happened on 9/11, it was much more a real concern than it had ever been before. Reporter: Norton even prepared her possible speech to the American public. I didn't write anything on paper, but I certainly thought about it. Reporter: According to 2004 designated staffer Alex Vogel, yes, there are staffers in those bunkers too, there are some perks. We were very well fed. I think we had steak and lobster and red wine, which was great. They didn't want to us go hungry in the event of an apocalypse, which was somewhat reassuring. Reporter: There are several of these cloak and dagger hide-aways. This one in Colorado nicknamed the mountain. Cheyenne mountain air force station is 2,000 feet deep, hidden behind multiple 25-ton blast doors. Three feet thick. Five acres dug out of the rock. 15 buildings built to withstand a nuclear bomb. Supported on 1,300 of these half-ton steel springs. Like a shock absorber. Reporter: It has its own power plant, a 6 million gallon water supply, potentially ready to support not just the president but hundreds of staffers to help him. And then there's the secret entrance to another bunker in West Virginia. The wallpapered panel slides away revealing this. Welcome to the bunker. Reporter: The greenbrier bunker. The floor is five feet thick, the walls are five feet thick, the ceiling is five feet thick. We're in a big concrete box. Reporter: Underneath a posh ho hotel, this facility was built during the cold war to house congress in case the U.S. Capitol was destroyed by a nuclear bomb. These showers were for the congressmen to be decontaminated from nuclear fallout. Clothing would have gone in. It would have been burned in an incinerator in the power plant. Reporter: The nuclear fears of the cold war may be a thing of the past but each year a cabinet member is still asked to skip the state of the union address. They are then put in a secure location with a support staff before the state of the union begins. And they're brought back only the next morning. Reporter: Former U.S. Attorney general Alberto Gonzalez once served as the designated survivor during president bush's 2007 address to a joint session of congress. Madam speaker, vice president Cheney, members of congress, distinguished guests, and fellow citizens -- My FBI detail drove me to Andrews air force base. There were a group of individuals there from various departments and agencies. All carrying these black binders. Their job was to advise me in the event that I assumed the presidency. Then I settled in, in front of a large monitor, and watched president bush give his state of the union. Reporter: He talks about his experience in his new book "True faith and allegiance." It suddenly hit me in the middle of that speech somewhere, oh my gosh. Reporter: On the show the white house is destroyed in a murky terror attack. The show invited us to this warehouse turned Navy S.E.A.L. Training base. So this will be the area where you actually film the scene? He and I will be here. They'll be actually treading through the compound. Reporter: The action scenes are high octane. But pondering how an average Joe might react to suddenly becoming commander in chief -- Where are we going? Presidential emergency operation center. Reporter: Is what makes "Designated survivor" a thinking man's thriller. What do you think it takes to be a good president? I think maybe you have to try and not be a good president, you have to try and be a good person. Good night, daddy. Good night, little pea. In the pilot they bring out the nuclear football. You say, what's the code? Should I have an eye scan? Do you need my fingerprints or an eye scan or something? No, sir. It's not like the movies. The guy laughs and says, yeah, we don't do that. Reporter: The show going to great lengths to keep things realistic. We're here on the Toronto set of "Designated survivor." The cast and crew hard at work. All of this is a stand-in for a swanky neighborhood in Alexandra, Virginia. Every detail well thought out. All the way down to the Washington news van. The president's speechwriter a veteran actor with real-life white house cred. Mr. President. Maybe the country's just not ready yet. Reporter: Kal Penn, everyone's favorite stoner from "Harold and Kumar." That was the best meal of my life. Reporter: Famously left Hollywood to serve in the office of public engagement in the Obama administration. You can trust the working conditions inside the beltway and inside Hollywood? You can't, absurd. Oh, come on. I have an air conditioned trailer and somebody will bring me a coffee if I want it, here. There -- You're in cramped quarters. I'm eating vending machine sandwiches and working 19 hours a day. Reporter: On set, Penn became a de facto fact checker. If you get the kal Penn seal of approval that's a good thing? Flattering unthat ran it's little things. How many people would be in a particular office. Would somebody actually run into this person or are we taking creative license with it? That's weird. Reporter: While they might not get everything right, no doubt all the drama will feel true to life. For "Nightline" I'm juju Chang in Toronto.

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